Indicating instruments

ABSTRACT

An inclinometer comprising a sensing unit and a readout unit which in use are remotely connected together through a cable, the sensing unit including two inclination sensors with sensing axes arranged at right angles and associated electronic circuitry to provide output signals which are fed to the readout unit through the cable. Also, along each of three coordinate axes there is provided a magnetic sensor, and the readout unit comprises means for converting AC signals from the inclination and magnetic sensors to DC analog signals whereby both inclination and azimuth of said sensing unit are continuously provided.

llite States ussell atent I 1 Feb. 12, 1974 HNDICATING HNSTRUMENTS [75]Inventor: Michael King Russell, Cheltenham,

England [73] Assignee: Scientific Drilling Controls, Costa Mesa, Calif.

[22] Filed: June 9, 1971 [21] Appl. No.: 151,561

Related US. Application Data [63] Continuation of Ser. No. 764,157, Oct.1, 1968,

abandoned.

[52] 11.5. C1 33/312, 33/313, 33/350, 33/352 [51] lint. Cl G0lc 9/04,E211) 47/022 [58] Field of Search 33/205, 205.5 E, 312, 313, 33/350, 352

[56] References Cited UNITED STATES PATENTS 2,000,524 5/1935 Kothny etal. 33/205 2,466,687 4/1949 Craddock et al.... 33/204 FA X 2,597,1255/1952 Noxon 33/204 FA X 2,671,275 3/1954 Burns 33/204 FA 3,308,5493/1967 Porter 33/205 3,434,219 3/1969 Bowman 33/205 FOREIGN PATENTS ORAPPLICATIONS 544,410 1/1956 Belgium 33/205 911,242 5/1954 Germany 33/205538,499 1/1956 ltaly 33/205 545,175 6/1956 ltaly..... 33/205 141,5018/1953 Sweden 33/205 Primary ExaminerRobert B. Hull Attorney, Agent, orFirmNienow & Frater [57] ABSTRACT An inclinometer comprising a sensingunit and a readout unit which in use are remotely connected togetherthrough a cable, the sensing unit including two inclination sensors withsensing axes arranged at right angles and associated electroniccircuitry to provide output signals which are fed to the readout unitthrough the cable. Also, along each of three coordinate axes there isprovided a magnetic sensor, and the readout unit comprises means forconverting AC signals from the inclination and magnetic sensors to DCanalog signals whereby both inclination and azimuth of said sensing unitare continuously provided.

9 Claims, 9 Drawing Figures Pafented Feb. 12, 1974 5 Sheets-Sheet l H F4 INCLINATION TEST +5 ,w, AZIMU |NVENTOR M/a/Mn KIA 6 fussy/.1.

ATTORNEYS Patented Feb. 12, 1974 5 Sheets-Sheet 5 PIN ON 56 TIMING GAPTIMING (RESET) GAP 0V 1 A REF SAIIVPLERESET REE A2 SA/ZPLE REF 9SAGHERESET REF W2 SA/gPLE INVENTOR W/c/Ma ///V6 fwsa 4 ATTORNEYSPatented Feb.

5 Sheets-Sheet 4 DEMODULATOR K V A2 HG2 SECOND 2%; 05am DEMODULATOR) M'CONSTANT CIRCUIT GENERATOR SIGNAL COMPI XRATO COUNTER BISTABLE EYLEMENTINVENTOR M/a/Ma {we flaw? fyv/vy v UM ATTORNEYS lNDlCATllNG INSTRUMENTSThis is a continuation of application Ser. No. 764,157, filed Oct. l,l968 and now abandoned.

This invention relates to indicating instruments and in particular toinclinometers which provide an indication of the inclination of an axiswith which a sensing unit of the instrument isaligned.

The object of the invention is to provide an inclinorneter whichoperates electrically and provides, at a readout unit which is separatefrom a sensing unit and connected thereto by an electrical cable, anindication of the inclination being measured, the construction beingsuch that remote measurement is facilitated. Another object is toprovide such an instrument which is particularly suitable for measuringthe inclination and direction of a borehole such as is drilled whenprospecting for oil or mineral deposits, or in civil engineering work.

According to one aspect of the invention the sensing unit of aninclinometer includes two inclination sensors arranged at right anglesand associated electronic circuitry to provide output signals which arefed to a remote readout unit through a connecting cable. The use of twoinclination sensors in this manner enables the component inclinationvectors to be measured with respect to the reference axes of thesensors, and this enables calculation of the resultant inclinationvector and its angular relationship to the reference axes.

The sensors may be gravity sensitive to enable the inclination withrespect to a vertical zero axis to be indicated or determined, in whichcase they may employ strain gauges to determine the deflection incorresponding perpendicular directions of a weighted cantilever beam, oralternatively they may employ electrolytic switches of known type;. ineither case the strain gauges or electrolytic switches will normally beconnected in a bridge circuit. Alternatively each sensor may bemagnetically sensitive and take the form of a fluxgate, whereby tomeasure the components of the earths magnetic field along the fluxgateaxes, and as a result allow the angular relationship between theinstrument axes and the earths field vector to be determined It will beappreciated'that up to three sensors of one type can be usedrespectively providing output signals indicative of three mutuallyperpendicular coordinate measurements with respect toeither the earthsgravitational field or the earths magnetic field, and a particularlyversatile instrument results from a mixture of the two types of sensorsproviding four or 'five of the total number of six coordinatemeasurements which are available. ln a preferred embodiment, the sensinghead includes two mutually perpendicular strain gauge gravity sensorsand three mutually perpendicular fluxgate magnetic sensors.

The inclination measurements can also be used to determine the rotationof the sensing unit about the axis of the borehole and with respect toeither the gravitational or magnetic fields. This facility renders theinstrument particularly suitable for use in directional drilling of deepboreholes which may, for example, reach depths of 20,000 feet whenprospecting for oil. To this end the outer casing of the sensing unitmay be formed to locate above the drill head, after falling down throughthe drill tube, in only one relative angular position so that thelocated angular position of the sensing unit corresponds to that of thedrill head. The mathematical basis of the determination of theinclinations and angular position of the sensing unit, from the sensorreadings, is described later in this specification.

For measurements of relatively short boreholes, as met with in civilengineering for example, all'the electronic circuitry associated withthe sensors may be contained in the sensing unit, with the circuitryproviding d.c. output signals which are indicated by a suitable meter inthe readout unit. These d.c. signals are conveniently voltages withrespect to a common ground, and such use of d.c. transmission with amulti-core cable facilitates remote indication as the inductive andcapacitive characteristics of the cable present no problem. Theindicating voltmeter in this case preferably provides a digital readout,and the readout unit may have a rotary selector switch which connectsthe different output signals to the meter in turn.

For deep boreholes, down to say 20,000 feet, d.c.

transmission along a multi-core cable becomes economically undesirabledue to the high cost of a suitable cable. This problem may be overcomeby using single core cable, preferably with a conducting outer sheath,and employing pulse code transmission of the sensor data with the cablealso carrying a d.c. power supply current to the sensing head. So thatthe voltage drop along the cable will be compensated a current regulatorcircuit feeding the cable and in turn feeding the head circuits, may beprovided in the readout unit. Repetitive series of pulses may betransmitted and separated by a synchronising time gapQeach seriescommencing with a reference or marker pulse and the spacing of twoindividual pulses being indicative of the corresponding sensor signal.

In a portable instrument the readout unit meter preferably housesbatteries which energise the instrument and are connected to the sensingunit through the cable, and for measuring the inclination of boreholesthe sensing unit is desirably housed in a non-magnetic cylindrical casewhich is slightly less than the minimum diameter of borehole with whichthe instrument is to be employed. An aluminium alloy case may be usedwhich for maximum protection against scratching and corrosion has a hardanodic finish, and which is fully sealed to allow operation in water.For accuracy the case should not be an unduly sloppy fit in theborehole, and a range of adaptors may-be provided which fit on the caseand increase its effective diameter for use with various sizes ofborehole. For deep boreholes an outer pressure-resistant metal casingmay be employed, again of a non-magnetic material so as not to interferewith the magnetic measurements.

As described the sensing unit preferably includes a magnetic sensor inthe form of a fluxgate, conveniently two or three mutually perpendicularfluxgates, which are fixed with respect to the measuring axis of thesens ing unit. Fluxgates as previously employed in instrument work havenot been arranged in this manner but have seen embodied in pendulousfluxgate assemblies so that under the influence of gravity theassemblies take up a position which enables the magnetic fieldorientation to be determined only in a horizontal plane. Thus accordingto another aspect of the invention an inclinometer has a sensing unitwith a measuring axis the inclination of which is indicated on or can bedetermined from the indications on a remote readout unit and whichincludes an'inclination sensor which is responsive to the local magneticfield and takes the form of a fluxgate fixed with respect to themeasurement axis.

The use of three mutually perpendicular fluxgates enables the resultantof three magnetic field vectors to be computed, thus enabling the localmagnetic field to be determined. Although this may be desirable in somecircumstances, the arrangement has a more significant advantage in thatthe sum of the squares of the three fluxgate measurements should beconstant unless the magnetic field changes. Thus a running check can bekept on the constancy of the local magnetic field if there is any changeit is reasonable to assume that there is also distortion and hence thatthe directional readings based on magnetic measurements are likely to beunreliable.

The invention will now be further described with reference to theaccompanying drawings which illustrate, by way of example, two boreholeinclinometers one of which is designed for use with relatively deepboreholes, for example to a depth of any 20,000 feet. In the drawings:

FIG. I is an external view of the complete instrument of the more simpleembodiment,

FIG. 2 is a partially cut-away view of a sensing unit of thisinstrument, showing the general internal arrangement in a somewhatdiagrammatic manner,

FIG. 3 is a corresponding block circuit diagram,

FIGS. 4 and 5 are sub-circuit diagrams, partially in block form,

FIG. 6 is a view fundamentally similar to that of FIG. 2 butillustrating the second and more sophisticated embodiment for use withdeep boreholes, the sensing unit being illustrated in an operativeposition within a drill collar, and the head 64 being shown disengagedfrom the slot 65 of casing 57,

FIG. 7 comprised of FIGS. 7A and 7B is a somewhat diagrammatic showingof the main current components of this second embodiment, mainly inblock form, and

FIG. 8 illustrates typical waveforms illustrative of the pulse codedtransmission of information which is used between the sensing unit andthe remote readout unit of this second embodiment.

The instrument of FIGS. 1 to 5 includes a sensing unit 1 with acylindrical aluminium alloy casing 1a which can be dropped down theborehole, the inclination of which is to be measured, and is connectedthrough a six-cored cable 2 to a remote readout unit 3. The latter unitembodies a voltmeter providing a digital display at 4 and a rotaryselector switch 5 which has two test positions and four measuringpositions in which four of the cable cores are respectivelyconnected tothe voltmeter. The casing of the readout unit houses dry batteries whichsupply the sensing unit 1 through the other two cores of the cable, anda press to read" switch 6 is associated with the voltmeter. The digitaldisplay 4 gives easier reading than a conventional moving coil meter andin addition is less likely to be damaged by mechanical shock.

The instrument is designed to be used with nominally vertical boreholes,and at the top or cable end the unit 1 has a threaded bore 7 for fittinga ring bolt for the attachment of a wire cable (not shown) on which itis lowered into the borehole. The sensing unit 1 provides four outputsignals each of which is dependent upon the inclination of thelongitudinal axis A A1 of the unit 1, which axis can be considered asthe measurement axis. Two gravity sensitive sensors of the unit 1 in theform of electrolytic switches 8 (FIGS. 2, 3 and 4) provide two derivedsignals which are indicated in the respective switch positions as anglesA 1 and A 2 of the vertical direction with respect to the two mutuallyperpendicular and nominally horizontal axes with which the electrolyticswitches are respectively aligned. These switches allow inclinations ofup to 5 to be indicated to an accuracy within 103.

Two mutually perpendicular fluxgates 9 (FIGS. 2 and 5) within thesensing unit 1 provide azimuth output signals, indicated respectively inthe other two switch measurement positions ill, and (11 which arerepresentative of the intensity of the magnetic field along two othermutually perpendicular axes which are also perpendicular to themeasurement axis. The two resultant azimuth readings enable the angularposition of the measurement axis with respect to magnetic north to becalculated by determining the vector resultant. The fluxgates aremounted in the casing 1a as a subassembly, which is indicated at 9a inthe block circuit diagram of FIG. 3. The magnetic field and inclinationmeasurements are taken along parallel axes, i.e., the two fluxgates 9are respectively aligned with the electrolytic switches 8. Theassociated electronic circuitry is illustrated diagrammatically as ablock 10 in FIG. 2, this being operative to provide d.c. output signalsrepresentative of the gravity and magnetic measurements, and theelectrolytic switch and fluxgate circuits will now be described withreference to FIGS. 3 to 5 merely in sufficient detail for theiroperation to be understood.

Each electrolytic switch 8 (see particularly FIG. 4) takes the form of ashort curved and sealed tube 11 partially filled with the electrolyte 12and arranged with the air space 13 at the top similarly to a bubblelevel. End wire electrodes 14 project through the electrolyte l2 andwhen the tube 11 is level, as shown in FIG. 4, have equal lengthsprojecting into the air bubble 13. A central electrode 15 which isarranged symmetrically with respect to the electrode 14 is arranged atthe bottom so that it is always immersed in the electrolyte 12. Theelectronic circuitry 10 includes a power oscillator Oscl with afrequency of l kcs. which energises the two electrolytic switch circuitsand also the two fluxgate circuits. Each electrolytic switch 8 isconnected in a bridge circuit energised from the oscillator Oscl throughan isolating transformer T1 and equal series resistors R1 and R2. Twoarms of the corresponding bridge are provided by the impedances betweenthe central electrode 15 and the two end electrodes 14 respectively, andthe other two arms are provided by fixed equal resistors R3 and R4. Thejunction between R3 and R4 and a centre tapping of the secondary windingof T1 are connected by a line 16 to the dc. power supply, and thecentral electrode 15 is connected through an isolating capacitor C1 tothe input of an ac amplifier Al the output of which forms the input of aphase-sensitive demodulator DMl which is supplied via line 17 with areference voltage from the power oscillator, the latter voltage thusbeing in phase with the bridge energising voltage. The resultant outputof the demodulator DMI is a dc. voltage A proportional to theinclination of the electrolytic switch 8, and the circuit constants arechosen in relation to the sensitivity of the digital voltmeter 4 so thata direct angular reading is provided by the latter. In the block circuitof FIG. 3 each demodulator and amplifier is shown as a unit 18.

Each fluxgate 9 comprises a pair of parallel elements aligned with thefluxgate axis and in the form of magnetic mumetal rods 19 positionedwithin energising coils which are connected in series and form two armsof a bridge circuit; the relatively remote ends of the coils areconnected to the two ends of a secondary winding 22 of a matchingtransformer T2 driven by the power oscillator Osc l. A centre tapping ofthe winding 22 is earthed, so that the two halves thereof form the othertwo arms of the bridge, and the common ends of the coils are connectedto the input of an a.c. amplifier. If neither of the mumetal cores 19are saturated there is no bridge output fed to the amplifier A2 as thebridge is balanced. With one core 19 only saturated there is a bridgeoutput as the corresponding energising coil 20 is effectively resistivewhereas the other is inductive, and after both cores l9 saturate bothcoils 20 are effectively equally resistive and again there is no bridgeoutput. As the two cores 19 are connected in magnetic opposition theportion of each energising cycle for which only one is saturated willdepend upon the magnetic field strength along the fluxgate axis. Thusthe width of the bridge output pulses is a function of the magneticfield strength along that axis. The applied voltage is chosen to besufficient to drive the cores 19 into saturation approximately halfwaythrough each half-cycle, and as saturation accurs twice per cycle theoutput has a strong second harmonic component. The main second harmoniccomponent of the input to the amplifier A2 is amplified thereby and fedto a phase-sensitive demodulator DM2 which is supplied with a secondharmonic reference voltage from a second harmonic generator HGI poweredat 23 by a reference voltage derived from the power oscillator Osc l.The resultant output of the demodulator DM2 is again a d.c. voltage I]!which in this case is proportional to the intensity of the magneticfield in the direction of the fluxgate axis. In respect of the fluxgatecircuits, each demodulator and associated amplifier is illustrated as aunit 24, the generator HGl being common to both these circuits.

As the outputs of the demodulators DMI or DM2 are phase sensitive theirsign, i.e., positive or negative, enables the angular quadrants in whichthe inclinations lie to be determined with respect to fixed instrumentcoordinates. The resultant inclinations, with respect to the verticaland the earths magnetic field, can be determined by simple graphicalresolution if the local value of the earths magnetic field is known.

As the instrument of FIGS. 6 to 8 is intended foruse down to depths ofthe order of 20,000 feet the transmission of d.c. signals up amulti-core cable is no longer economical, and as an alternative, pulsecoded transmission of the sensor information is used along a centralcopper core 50 of a cable 51 by which the sensing unit 52 is in usesuspended. This cable has an outer steel sheath in the form of adouble-wrapped rope which is grounded and insulated from the core 50 bynylon insulation. The core and sheath are also used for the transmissionof d.c. power to the sensing unit 52, this being supplied from aconstant current power supply 53 in the remote readout unit 54 to whichthe upper end of the cable 51 is connected. Thus the sensing unit issupplied with a constant supply current whatever the effective lengthand resistance of the cable 51, and the power supply for the sensingcircuits is provided at a constant voltage as the sensing circuits areof constant resistance, and these circuits are supplied from the cable51 through a resistor R5. This resistor presents to the output signalsan adequate impedance and the cable is not short circuited to the signalpulses. At all positions other than full depth the cable 51 is partlywound on a drum, and hence the effective d.c. resistance of the outersheath is reduced.

The first embodiment was developed from the viewpoint of measuring theinclination and azimuth direction of the borehole, whereas the FIGS. 6to 8 embodiment was further developed on the basis that the samemeasurements can be used to determine the angular position or roll angleof the sensing unit with respect to the axis of the borehole. It isimportant to be able to determine this angular position when employingdirectional drilling, whereby to achieve the appropriate positioning ofthe directional drilling tool. To this end it is important to ensurethat the sensing unit 52 will locate within the drill collar 56 (FIG. 6)in one position only. An alumium casing 57 of the sensing unit iscontained within a pressure casing 58 of non-magnetic material, thisouter casing being necessary in view of the hydraulic pressureencountered at the depths concerned, and the casing 58 has a leading end59 of shoe-like formation presenting inclined guidesurfaces which leadto a locating slot 60. When the sensing unit is dropped into the drilltube it falls down the latter and enters the drill collar 56, which isalso of non-magnetic material, and the end 59 engages an inwardlyprojecting pin 62 on the collar 56. This results in the casing 58 beingguided so that the pin 62 enters the slot 60 to fix the angular locationof the unit 52 within the collar 56. To protect the sensors fromexcessive shock resulting from deceleration of the pressure casing 58 asthe latter seats home in the drill collar 56, the inner casing 57 isconnected to the pressure casing 58 through a shock absorber assembly63. This has a T head 64 which engages a reference T slot 65 at theforward end of the casing 57, whereby to ensure correct angularalignment of the two casings.

Two gravity sensitive sensors are again provided, but in this case theyutilise four strain gauges 66 respectively associated in pairs in bridgecircuits as shown in FIG. 7. These strain gauges detect the deflectionof a pendulous cantilever beam 67 which is aligned with the measurementaxis, i.e., the longitudinal axis of the sensing unit 52, and which hasa reduced neck portion 68 of square cross-section. The two strain gaugesof each pair thereof are respectively attached to opposite side faces ofthe square section neck, whereby to provide a bridge output dependent onthe deflection of the beam in a corresponding direction at right anglesto the measurement axis. Thus, the two pairs of strain gauges v66provide for measurement of two components of inclination disposedmutually at right angles. The beam 67 is firmly anchored within thecasing 57 at the end adjacent the reduced neck 68, and at the other enda weighted pendulum portion 69 has a surrounding rubber buffer ring 70which has clearance with the casing 57 and engages the latter to preventundue deflection of the beam 67 under shock forces. It will be seen fromFIG. 7 that as with the earliest embodiment each strain gauge sensorprovides a d.c. output signal A the bridge being supplied from a 1,000c.p.s. power oscillator Osc2 through an isolating transformer T3, eachcircuit including an a.c. amplifier A3 and demodulator Dm3.

Three mutually perpendicular fluxgates 72 are now mounted side by sidealong the measurement axis within the casing 57. As before the fluxgatesare driven from the oscillator Osc2, the fluxgate bridge output beingamplified by an amplifier A4 and demodulated by a phase-sensitivedemodulator Dm4 supplied with a second-harmonic reference voltage from asecondharmonic generator I-IG2 powered from a reference voltage derivedfrom the power oscillator. As a refinement the demodulator d.c. outputsignal is applied to the input of a high gain d.c. amplifier AS theoutput of which is fed back to the fluxgate via a feedback resistor R6.The feedback current produces a magnetic field in the fluxgate which isin opposition to the external magnetic field, and due to the externallyhigh gain of the amplifier A5 a stable condition is reached with thecurrent feedback field substantially equal and opposite to the externalmagnetic field. As the feedback field is dependent solely on thefeedback current and the feedback resistor R6 is fixed, the outputvoltage of the amplifier A5 is proportional to the external magneticfield component aligned with the fluxgate axis. Three identical circuitsof this nature are provided respectively associated with the threefiuxgates 72, the advantage of this arrangement being that themeasurement scale factor depends only on the fluxgate winding geometrywhich determines the feedback field for a given feedback current and thevalue of the feedback resistor. Both these elements are very stable andhence the accuracy of the magnetic sensor measurement signals 1111, i112and i113 is extremely high and stability is unaffected, for example, byany drift in amplifier gain.

The object of the pulse code transmission circuit, the main componentsof which are illustrated in block form in FIG. 7, is to produce a trainof pulses with the time between the pulses proportional to the d.c. Aand it: signals. As the dc. measurement voltages may be either negativeor positive, it is essential to be able to distinguish the dc. signwhile the time intervals must of necessity be positive. In order toachieve this the circuit defines a reference time gap which correspondsto a zero d.c. signal, time intervals shorter than this definingnegative d.c. signals while time intervals longer than the referencetime define positive d.c. signals.

The basic elements used to define the time intervals are a linearvoltage ramp and signal comparator circuits, each time interval beingthe time taken or the voltage to rise from a constant negative referencelevel, for example 4v, to the A or I: voltage level concerned. In orderto transmit the pulsed information sequentially the dc. signals areselected in turn by means of a solid state switch, which is of a formknown per se, and which selects ten inputs in turn. As marked in FIG.7A, the first and sixth inputs are connected to earth, the fifth, ninthand tenth inputs are each supplied with a constant signal of +4v, andthe sensor A and ill outputs are connected to the other five inputs. Asweep capacitor C2 is supplied from a constant current generator CGl sothat the capacitor voltage increases linearly with time from thenegative reference level until it exceeds the input signal levelselected by the switch 73. When it exceeds the signal level an upperlevel signal comparator 74 operates, and this toggles a bistable element75 which in turn operates a switch 76 which discharges the sweepcapacitor C2. When the sweep capacitor reaches the negative referencelevel a lower level signal comparator 77 operates and resets thebistable element 75, thus opening the switch 76. As the discharge timeof the capacitor C2 is very much less than any of the charging times anoutput at 78 from the bistable element circuit appears as a narrowpulse, which is the coding system output, the spacing of consecutivepulses being indicative of the corresponding charging time and hence theinput voltages selected. A typical pulse train is illustrated at P inFIG. 8, and the pulses are supplied to a power driver stage 79 to obtainthe line pulses which are fed to the cable 51 through a couplingcapacitor C3.

The pulses at the bistable output 78 are also fed to a counter 80associated with a decoder 82 which controls or steps on" the selectorswitch 73 so that the production of a pulse at the same time as thecapacitor C2 discharges results in the next input signal being selected.The decoder 82 also provides an overriding control for the currentgenerator CGl which charges the capacitor C2, and on each tenth timeinterval the charging current is reduced by a factor of three and the+4v signal is connected to the upper level comparator 74. Thus the sweepvoltage rate is reduced by a factor of 3, and as the dc. sensor outputsignals have a nominal full scale value of i 3v the resultant timeinterval is at least three times as long as any of the measured signalintervals and can be used as a synchronising interval to synchronise thereadout decoding equipment.

The use of the constant current power supply 53, with the inherent highinternal impedance associated with a constant current source, enablesthe signal pulse to be taken off from the upper end of the cable througha simple coupling capacitor C4, and the pulses are fed to both a counter83 and a pulse gap detector 84. The circuit of the detector 84 operatesas a pulse generator which provides an output pulse at a fixed timeinterval following an input pulse, and if an input pulse arrives beforethe output pulse has occurred the pulse generator is restarted. Thus foras long as pulses arrive at the gap detector 84 more frequently than theinherent fixed time interval, i.e., the time delay between receipt of apulse and the occurrence of an output pulse, then no output pulse willbe provided. The gap detector 84 is connected to the counter 83 througha reset line 85 and it operates to reset the counter 83 in order tosynchronise the latter with the pulse generator circuits in the sensingunit. To this end the inherent time delay of the gap detector 84 isapproximately double that of the longest signal pulse interval providingin the pulse code except the timing gap. As described, each timing gapin the pulse code is about three times the maximum signal interval, andthe result is thus that the gap detector 84 provides a pulse about twothirds of the way through each timing gap, and this pulse is used toreset the counter 83 to the tenth position in the pulse series. Thecounter 83 drives a decoder circuit 86 and the counterdecoder systemproduces outputs corresponding to the condition of the sensing unitcounter 80; these outputs are available to a manual signal selectionsystem 87 which determines the functions that these outputs are tofulfill, as will now be described.

In order to take survey measurements, ie to read outputs for the purposeof calculating the borehole inclination and azimuth angles, thetransmitted signals are measured as follows. In the appropriate positionof a rotary selector switch the signal selection system 87 connects oneof the time interval decoding outputs of the decoder 86 to a gatecircuit 88 controlling a standard free-running oscillator Osc3. Theoutput of the gate 88 is connected to a counter and decimal readoutdisplay unit 89. Thus a digital readout is provided of the number ofstandard oscillator counts during the time interval corresponding to thesignal selector, and standard resetting circuits are employed to preventthe counts adding in the counter of the unit 89. Thus the indicatedcount is representative of the instantaneous signal time interval.

The rotary selection provided by the system 87 allows any one of thetransmitted signal intervals to be indicated as a count by the unit 89,so that the six time intervals concerned five sensor signals and thereference signal are read off as decimal numbers and can form the basisfor a survey calculation. As already mentioned the reference signalpulse gap corresponds to zero sensor signals, and thus the referencesignal readout is subtracted from the sensor signal readouts to give thetrue positive or negative value of the A and ill values.

The instrument provides for analogue computation based on thetransmitted sensor signals, and in order to achieve this, d.c. analoguesof the transmitted signals must first be obtained. This is achieved bymeans of a switched integrator circuit embodied in the signal selectionsystem 87, the fundamentals of this integrator circuit being illustratedat 90. During the reference time interval a positive supply voltage isconnected by an electronic switch 92 to the input of the electronicintegrator 93 causing it to slew in a negative sense at a constant rate.At the end of this time interval the integrator thus has a negativevoltage proportional to the reference time interval, and during one ofthe selected signal time intervals an equal and opposite supply voltageis switched by means of a switch 94 to the integrator 93 causing it toslew in the positive sense. Thus, at the end of the signal time intervalthe integrator voltage is proportional to the difference between thereference and signal time intervals, and is therefore proportional tothe corresponding transmitted signal voltage. Sampling switches 95enable the integrator output voltages.

to be stored in storage capacitors C as d.c. analogues of thetransmitted signals, and the integrator is automatically reset to zero,by a reset switch 97, prior to a decoding sequence. FIG. 8 illustrates,at IA and I respectively, the integrator output when the A and ll!signals are being decoded.

In a simple form of the circuit only one switch 95 and storage capacitorC5 need be provided, at any time the integrator operating on only onesignal interval. However, the instrument as illustrated is slightly moresophisticated in that, as shown in FIG. 7, two sample switches 95 andstorage capacitors C5 are provided. The reference time interval istransmitted twice in each complete pulse cycle, at the first and sixthswitch positions, and by appropriate control of the switching sequencesa pair-of signals are obtained from the same integrator 93 by employingtwo decoding sequences per cycle. Two reset intervals per cycle areused, at the fifth and tenth positions respectively. Thus the circuitcan be used to provide simultaneous analogues of A1 and A2, i.e., thegravitational strain gauge sensor outputs, or alternatively i111 and1112 which are two of the magnetic fluxgate outputs. From a knowledge ofAl and A2 it'is possible to determine the roll angle which in theanalysis which follows will be identified as d) For certain computationsit is necessary to obtain knowledge of a quantity such as lb aA where ais a constant for a given geographical location. To this end a secondpair of input switches which duplicate the switches 92 and 94 areprovided for the integrator 93, and these are appropriately switched sothat the pairs of signals which are stored in the capacitors C5 are eacha combination of two signals in the desired form. As shown in FIG. 7 theswitches 92 and 94 feed the integrator 93 through a resistor R7, and theresistor R8 which is also shown is associated with the other pair ofswitches when it is desired to obtain combination output signals. If R8R7/a then the outputs will be 1111 oz. Al and 1112 0:. A2, which asshown later can be used to determine 4: ()5

A resolver servo 98, which is a device already known in otherapplications, is provided in the readout unit 54 to provide a directindication of rotational angles mechanically computed on the basis ofthe integrator output signals. The resolver 98 takes the normal form ofan a.c. machine with a pair of stator and a pair of rotor windings, thetwo windings of each pair being arranged physically at to each other andfunctioning as follows. If two a.c. voltages V1 and V2 are applied tothe stator windings and the rotor is turned until one of its windingshas zero voltage output then the angle of the shaft of the rotor withrespect to the stator reference sssitisayill h is M2. Y1. @1.t 19. $a92111? other rotor winding will be proportional to V V1 +V2 In thepresent circuit the two d.c. servo input voltages obtained from theintegrator, namely Al and A2 (or llll at. Al and \l/ 2 +01. A2) areconverted to a.c. voltages by two 1,000 c.p.s. modulators M1 and M2, andthese two voltages are respectively connected to the stator windings ofthe resolver 98. One of the rotor output windings has its output voltageshifted in phase by 90 by a phase-shift network 100 and then amplifiedby a servo amplifier A6, the amplified and phase shifted voltages beingapplied to the control winding of a twophase servo motor M. Thereference phase winding of the motor M is supplied from a 1,000 c.p.s.power oscillator Osc4 which also drives the modulators MI and M2. Themechanical output of the motor M is connected, via a reduction gear box101, to the rotor shaft of the resolver 98, providing a mechanicalfeedback which operates to turn the resolver rotor to a position inwhich a zero output voltage is fed to the servo amplifier. The readoutdisc 99 is also connected to the shaft of the resolver to indicate theangle directly, and the other rotor winding of the resolver is connectedto a demodulator DM5 to give a d.c. output proportional to the amplitudeof the resultant vector, i.e. V V1 V2 A switch 102 can be turned to aSURVEY position for survey indications by the readout unit 89, to an IN-DIRECT position or to a DIRECT position. In the IN- DIRECT position thecounter and readout unit 89 can provide a readout which approximates tothe inclination angle for angles of less than 20. As will be clear fromthe following mathematical analysis, for such small angles 0 z /A1 A2and this readout is obtained in the INDIRECT switch position with thetwo A inputs to the resolver servo. The rotor output voltage of thelatter is demodulated by a 1,000 c.p.s. demodulator DM5, the demodulatedoutput being supplied by a gate-control generator circuit 103 whichoperates on the gate 88 to provide a gate width proportional to the 3,79 1 ,043 1 1 l2 demodulated signal. Thus the readout unit 54 count isalso proportional to this signal, and hence approxi- G 0 mately also to6 for values thereof less than 20. When the switch 102 is in the DIRECTposition the resolver Servo is computing from the functions 11 A1 and 25 and the gravity vector referred to the instrument axes or A In thisposition the resultant resolver output be V V, V} is not used, and thedigital angle readout g is w o as developed below. I

For the purpose of completeness a mathematical G analysis will now begiven of the bias computation of the resultant roll angle, pitch angleor inclination, and heading angle or true azimuth reading utilising theseng1 0 sor outputs. [B] 0 l. Definition of axes The system of axesdefined by suffix 0" is earth gz g fixed with OX, horizontal anddirected towards maghi h h l i li d out i netic north, 0Y horizontal anddirected towards magg: 8 i 0 cos (1) netic east, 02,, vertical anddirected upwards.

Axes OXYZ, with no suffix, are fixed in the body of g i 9 i d, (2) thesurvey instrument.

2. Definition of rotations g: COS 9 Starting with OXYZ and OX Y Zcoincident, OXYZ is rotated about axis OZ by an angle ill which will bedefined as the inclinometer axis AZIMUTH AN- GLE.

Measurements of the components of gravitational force on a mass in theinstrument can yield quantities A,, A and A where From its new position,OXYZ is further rotated by g the PITCH ANGLE 0 about the axis OY. A2 21/and A a g,

The final rotation is again about the axis OZ by an angle which will bedefined as the ROLL ANGLE The angles 4) and 6 can then be determinedbymeans of the following expressions:

3. Co-ordinate transformation matrices 1 gu/gx) -1 Az/Al) (4) fromstandard classical mathematics of co-ordinate transformation, therelationship between any vector A in the earth-fixed fram and th v c rie e same e tor A 0: tan 1 (g zgv2 g in the instrument frame is XFTBTTZ'tan 1 Me s where [B] is a three by three matrix defined by the threerotations as follows:

cos #1 sin 111 0 cos 6 0 sin 6 cos d) [B]=lisin|l1 costb OIH O l 0:Hsino 0 0 1 sin 6 0 cos6 0 (cos 1!; cos 6 cos 4 sin 4/ sin 4)) [(sin|11cos6cosd +cos 1p sin (1)) Altemately, A [B]- -A Note: for conformaltransformations [3] [B]' where [B] is the inverse of [B] and [B]* is thetranspose of [B].

Thus:

ZI= [(-cosrbcosOsinda-sin 111605 11)) cossin6 4 The Gravity Vector ininstrument axes Let the gravity vector referred to the earth-fixed axes(Note: Equation (5) is the one mechanised in the deall ( cos 4: cos 6sin (I) sin up cos (1)) sin 1 cos (b sin 1/) -l cos ill cos 4;)

sin 6 sin 1!) (sin ill sin 6) cos 6 scribed instrument to yield (11.) 6.The Earths Field Vector in instrument axes If the horizontal andvertical components of the 5 earths field are H, and H, respectively,then the field vector referred to the earth-fixed axes is (sin 6 sin qb)cos 6 Let the field vector referred to the instrument axes be i1 F1 h 2then h,, B H.

which when multiplied out gives:

Measurements taken in the instrument yeild three quantities 111 111 and111 where ll! a:

and \11 a h From equation (1) and (2), we derive A a sin0cos A a-sin0sind Substituting in equations (9) and yields For pitch angles (0)up to 20 the approximation :1: z y gives less than 2 of error.

I claim:

1. An inclinometer comprising in combination;

a sensing unit comprising a casing having a pair of inclination sensorsoriented in fixed relation to the unit and at right angles to each otherand a magnetic sensor arranged along each of three space co ordinateaxes fixed relative to the unit;

means for shock mounting said sensors within said casing;

a cable;

means for delivering to said cable AC signals derived from the outputsof said inclination and magnetic sensors;

a readout unit for location remotely of said sensing unit and connectedthereto by said cable;

said readout unit comprising means for converting said AC signals to DCanalog signals to permit continuous computation of both inclination andazimuth of said sensing unit.

2. An inclinometer according to claim 1 comprising switch means forenabling each of said five sensors to be read independently.

3. An inclinometer according to claim 2 wherein said readout unitcomprises a counter and decoder.

4. An inclinometer according to claim 3 wherein said readout unitcomprises an electronic integrator for developing a voltage which isproportional to the signal being afforded by any one of said sensorsselected by said switch means.

5. An inclinometer according to claim 4 wherein said shock mountingmeans is connected between said sensors and said outer casing.

6. An inclinometer according to claim 5 comprising means for orientingsaid sensors in a known position in a remote location.

7. An inclinometer according to claim 6 wherein said inclination sensorscomprise strain gauges disposed at right angles to each other and whichare associated with a weighted pendulum for sensing gravity.

8. An inclinometer according to claim 7 wherein each of said magneticsensors is a fluxgate for determining magnetic field strength along arespective one of said three axes.

9. An inclinometer according to claim 8 wherein said casing for said.sensing unit comprises a non-magnetic pressure casing for enabling saidsensing unit to withstand relatively high hydraulic pressures.

1. An inclinometer comprising in combination; a sensing unit comprisinga casing having a pair of inclination sensors oriented in fixed relationto the unit and at right angles to each other and a magnetic sensorarranged along Each of three space coordinate axes fixed relative to theunit; means for shock mounting said sensors within said casing; a cable;means for delivering to said cable AC signals derived from the outputsof said inclination and magnetic sensors; a readout unit for locationremotely of said sensing unit and connected thereto by said cable; saidreadout unit comprising means for converting said AC signals to DCanalog signals to permit continuous computation of both inclination andazimuth of said sensing unit.
 2. An inclinometer according to claim 1comprising switch means for enabling each of said five sensors to beread independently.
 3. An inclinometer according to claim 2 wherein saidreadout unit comprises a counter and decoder.
 4. An inclinometeraccording to claim 3 wherein said readout unit comprises an electronicintegrator for developing a voltage which is proportional to the signalbeing afforded by any one of said sensors selected by said switch means.5. An inclinometer according to claim 4 wherein said shock mountingmeans is connected between said sensors and said outer casing.
 6. Aninclinometer according to claim 5 comprising means for orienting saidsensors in a known position in a remote location.
 7. An inclinometeraccording to claim 6 wherein said inclination sensors comprise straingauges disposed at right angles to each other and which are associatedwith a weighted pendulum for sensing gravity.
 8. An inclinometeraccording to claim 7 wherein each of said magnetic sensors is a fluxgatefor determining magnetic field strength along a respective one of saidthree axes.
 9. An inclinometer according to claim 8 wherein said casingfor said sensing unit comprises a non-magnetic pressure casing forenabling said sensing unit to withstand relatively high hydraulicpressures.